The present invention relates to a fluidized bed reactor and, more particularly, to a fluidized bed reactor useful in a method for the production of high purity polycrystalline silicon.
The use of silicon for electronic applications requires the production of ultra-high purity silicon material. For use in semiconductors, it is common to require silicon material with impurity levels of less than 1 part per billion (ppb). Continuing advances in the electronics industry and the development of many new products in this field have led to an expanding market for ultra-high purity silicon. High purity silicon is also required for the fabrication of solar cell arrays for the direct conversion of sunlight to electricity. For all such applications, improvements in existing silicon technology are urgently needed to achieve enhanced silicon purity and quality at reduced costs. The production of high purity polycrystalline silicon on a continuous or semicontinuous basis by use of a fluidized bed reaction zone is an important aspect of the overall processing to produce ultra-high purity, single crystal silicon.
The production of polycrystalline silicon from silane and/or halosilane containing gases is accomplished by the pyrolysis of the silane containing gas to high purity silicon.
The pyrolysis step utilizes a fluid bed reactor zone wherein the silane containing gases are passed through a heated fluidized bed of silicon particles. Within this fluidized bed the silane is decomposed into high purity silicon and hydrogen gas by-product. The high purity silicon is deposited onto the silicon particles of the fluidized bed. The deposition of the silicon onto the silicon seed particles causes the particles to grow in size. The larger product particles of silicon are then removed from the fluidized bed by conventional means. Such production processes are disclosed in U.S. Pat. Nos. 3,979,490 to Dietz et al., 3,012,861 to Ling and 3,012,862 to Bertrand et al.
The decomposition of silane to silicon occurs both heterogeneously and homogeneously. The homogeneous decomposition results in the production of silicon powder of submicron and/or micron size. The silicon in powder form exhibits high surface areas and, therefore all the particles in the fluidized bed tend to grow much slower compared to the rate of growth when the powder particles are not present in the fluidized bed. As the process continues, the smaller particles begin to accumulate in the reactor as they are produced homogeneously. As the amount of silicon powder increases, the product size silicon particles are produced at a progressively slower rate because of the reduction in the number of silicon particles available for growth. As the pyrolysis continues, the bed begins to segregate, with the smaller particles at the top and the larger particles at the bottom. As the larger product particles are removed from the bottom of the reactor, only large particles are removed. As the removal of large particles continues, eventually the entire fluidized bed reactor will become essentially a bed of smaller silicon powder particles.
The presence of the silicon powder particles during the subsequent single crystal pulling technique is undesirable since the particles do not readily melt, but rather, float on the surface and disadvantageously deposit around the newly formed single crystal rod. Further, the silicon in powder form is more subject to contamination during production due to the increased surface area. Another problem exists in the difficulty of handling micron to sub-micron size particles of silicon powder.
The homogeneous decomposition of the silane may be retarded by decreasing the temperature at which the fluidized bed reaction zone is maintained. However, this suffers from the drawback that the efficiency of the heterogeneous decomposition is also adversely affected by a substantial decrease in the silane pyrolysis temperature. Another remedy requires maintaining a low concentration of silane and/or halosilane in the reaction zone in order to limit the homogeneous decomposition. However, this remedy suffers from the drawback that a low concentration of silane/halosilane may result in a low production rate of silicon. Therefore, the need exists for a fluidized bed reactor which may pyrolyze silane and/or halosilane gases at temperatures which result in the efficient heterogeneous decomposition of the silane gases and accommodates the silicon powder produced by homogeneous decomposition in the fluidized bed by removing the silicon powder from the bed before it accumulates and causes the silicon powder particles to be predominant in the fluidized bed.
Other objects, advantages and features of the present invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings. It is understood that variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.